The present invention relates to control systems for multiphase a.c. machines, and more particularly to original processing systems for vector control of multiphase a.c. machines.
The main feature of main-flux-oriented control of M-phase a.c. machines is the transformation of the actual M-phase of (M-1)-phase (if M is uneven) and (M-2)-phase (if M is even) supply currents into orthogonal currents, one of which--a direct axis component--produces the main field and the other--a quadrature axis component--is a torque-producing current. The supply currents are controlled--in phase and magnitude--in such a way that the component producing the main flux and the torque producing component are controlled separately and thereby control the electromagnetic torque.
To perform the separation of the supply currents into the required two components, it is necessary to transform the stationary-axis supply-current components into two-axis current components of such a special reference frame that the direct axis of coaxial with the magnetising flux-linkage space vector.
Therefore, if in a stationary reference frame the M-phase magnetising flux linkage space vector is EQU .psi..sub.m =.vertline..psi..sub.m .vertline.e.sup.j.mu..sbsp.m ( 1)
where .vertline..psi..sub.m .vertline. and .mu..sub.m are the modulus and space angle of the magnetising flux-linkage space vector, the two-axis components of the supply currents (i.sub.x, i.sub.y) in the special reference frame are obtained from EQU i'=i.sub.x +ji.sub.y =ie.sup.-j.mu..sbsp.m =(i.sub.D +ji.sub.Q)e.sup.-j.mu..sbsp.m ( 2)
where i.sub.D and i.sub.Q are the two-axis currents in the stationary reference frame.
Since the direct axis of the special reference frame is coaxial with the magnetising-current space vector, in the special reference frame the quadrature-axis component of the magnetising-current space vector is zero and the electromagnetic torque will be proportional to the modulus of the magnetising flux space vector and the quadrature axis component of the supply currents (i.sub.y). Thus, the electromagnetic torque can be expressed as EQU t.sub.e .varies..vertline..psi..sub.m .vertline.iy (3)
In a first known system disclosed in European Patent Application No. 0121792, a vector control method and system for a three-phase a.c. induction motor is described. In this control system it is necessary to measure the rotor angular frequency of the motor and to know a number of parameters of the motor to enable the controls signals to be calculated, and furthermore the effects of saturation are not incorporated. In the present invention, however, the effects of saturation are taken account of. This is particularly important because it leads to better decoupling of flux and torque producing current components.
In a second known system disclosed in U.K. Pat. No. 1542024, a vector control system is described using voltage and current transformers to monitor the input voltages and currents of a three-phase a.c. induction motor. The input currents are differentiated in this system, several integrators are used and no allowance is made for saturation of the leakage flux paths, thereby leading to errors in the control.